1. Field of the Invention
The present invention pertains to wireless telecommunications, and particularly to point-to-multipoint transmissions in a wireless telecommunications network.
2. Related Art and Other Considerations
In a typical cellular radio system, wireless terminals, often called mobile stations or user equipment units (UEs), communicate via a radio access network to one or more core networks. The wireless terminals can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. Alternatively, the wireless user equipment units can be fixed wireless devices, e.g., fixed cellular devices/terminals which are part of a wireless local loop or the like.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the mobile stations within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a base station controller (BCS) node, sometimes termed a radio network controller (RNC). The base station controller supervises and coordinates various activities of the plural base stations connected thereto. The base station controllers are typically connected to one or more core networks.
One example of a radio access network is the Global System for Mobile communications (GSM) developed in Europe. Other types of telecommunications systems which encompass radio access networks include the following: Advance Mobile Phone Service (AMPS) system; the Narrowband AMPS system (NAMPS); the Total Access Communications System (TACS); the Personal Digital Cellular (PDS) system; the United States Digital Cellular (USDC) system; the wideband code division multiple access (WCDMA) system; and the code division multiple access (CDMA) system described in EIA/TIA IS-95.
In GSM systems the robustness of the radio signal is influenced by two factors: the modulation and coding scheme (MCS) and the transmit power. Higher transmit power makes the radio signal more error resilient but generates interference in the system, thereby disturbing other radio links. Furthermore, the available transmit power per base station is limited. Different Modulation and Coding Schemes use different code rates to protect the user data. MCSs with little parity provide high data rates but low robustness and are therefore chosen in case of good radio conditions. However, if the radio conditions are poor an MCS using stronger coding must be used at the cost of lower data rates seen by higher layers.
In a point-to-multipoint transmission mode, data (e.g., a video clip) is transmitted to several mobile stations on a common downlink channel (a so-called point-to-multipoint channel). Depending on the positions of the mobile stations in the cell and their respective radio conditions, each of the mobile stations will likely experience different link quality for the common downlink channel involved in the point-to-multipoint transmission. If the network is unaware of the position and conditions of the mobile stations, the coding scheme utilized for the point-to-multipoint transmission on the common downlink channel must be preconfigured to cope with a worst case link condition that could occur in that particular radio cell. This would guarantee sufficient quality for all mobile stations independent of their position in the cell. Unfortunately, also the mobiles with good radio link quality suffer from the decreased data rate provided on the point-to-multipoint channel. And even if all active mobiles in the cell experience good radio quality, radio resources are wasted by transmitting unnecessary parity information.
To increase the bit rate (depending on the actual radio conditions of all active receivers), the mobile stations could separately send information over individual feedback channels to apprise the network of their perception of the link quality of point-to-multipoint transmission over the common downlink channel. The network could then adapt the coding scheme to the actual link quality of the worst mobile station. Unfortunately, having individual feedback channels for each mobile station is impractical, especially if there are many mobile stations receiving the same data in a point-to-multipoint transmission.
What is needed, therefore, and an object of the present invention, is a technique for efficiently adapting the coding scheme for a common downlink channel carrying a point-to-multipoint transmission.